Earth Pressure with Narrow Backfill Zone

[Geo69]

Hello all, this is my first post. Looks like a great resource.

Anyway, I have a situation where a retaining wall is being constructed near a building foundation (about 4 to 10 feet away). I have been asked to provide lateral earth pressures under static and seismic conditions. The backfill between the retaining wall and foundation wall will be free-draining gravel (crushed stone).

Does anyone have experience with lateral pressures where there is a narrow backfill zone.

Any help would be appreciated.

Thank you.

 

[jdonville]

Geo69,

Your post did not clearly explain your situation -

Is this an existing building foundation? How high is the new wall? What is the elevation of the foundation bearing surface relative to the new wall?

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.

 

[Geo69]

Neither structure has been constructed. The retaining wall is planned to consist of a permanent soil nail wall approximately 14 to 26 feet in height. The soil nail wall is planned to be constructed at a horizontal distance of 4 to 10 feet away from the foundation wall of the proposed building. The soil nail wall is planned to allow for construction of the building but the structural engineer would also like to take advantage of reduced lateral pressures, if possible. The space between the soil nail wall and the building would be backfilled with gravel.

Has anyone had experience determining what the lateral pressures (static and seismic) would be in this situation?

Thank you.

 

[civilperson]

A reduction in lateral pressures could be obtained by NOT backfilling between the soil wall and building wall. Use compressible foam or corrugated paper forms. If a granular material is placed then the the movement of the soil wall will reflect the load on the building wall.

 

[AW12]

I wouldn't recommend compressible backfill because there may be consequences of ground movement beneath/adjacent to the building foundations.

Perhaps Geo69 could confirm the proposals for the building foundations and the type of founding soils? The soils beneath the building would also determine whether there is a reduced lateral earth pressure on the adjacent proposed retaining wall.

 

[jdonville]

Geo69,

This is my current understanding of the situation:

The (soil nail) retaining wall will be constructed first. The height of the wall will vary from 14 to 26 feet from a constant foundation grade. A wall foundation will be constructed at the same grade as the proposed soil nail wall in front of the soil nail wall. The distance between the two will vary from 4 to 10 feet.

You should design the soil nail wall (static case) as if there will be no backfill between it and the foundation wall, as the foundation wall will not be there during the soil nail wall construction. The foundation wall should be designed to withstand construction conditions - think of it as a bin that will contain whatever backfill you put between it and the soil nail wall. NAVFAC DM 7.2 page 77 shows static design pressures for compacted cohesionless fill (c=0) behind a retaining structure. A discussion of earthquake loading is presented starting on page 76.

Hope this is helpful.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.

 

[jdonville]

Geo69,

Just for future reference, you may want to post this kind of question in the 'Earth Retention Engineering' (Geotechnical) forum next time.

Welcome to eng-tips.

Jeff

Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.

 

[Geo69]

jdonville,

Yes, I should have used the earth retention forum.

I looked in DM 7.02 and am not sure which figure you are referring to; I may have an older version. Anyway, the foundation wall has been designed using at-rest pressures. The soil nail wall will also be designed as a permanent structure and will be built during excavation for the building. The structural engineer is wanting to use a reduced static and dynamic lateral pressure after the building is built and the space between the soil nail wall and building is backfilled. His thinking is that the soil nail wall will still be retaining the soil behind it and that the building will not "feel" the effect of the entire at-rest pressure because a portion of the wedge of soil which exerts the pressure will be retained by the soil nail wall. They are looking for the reduced pressure to save in steel and concrete for the building foundation.

 

[BigH]

It's an interesting question. My take on it is that the soil nail wall will be put in, will 'move' under the soil pressure behind it and hence realize a 'stable' state with respect to and prior to the retaining/foundation wall to be placed in front. The retaining/foundation wall would be subject to silo-pressure like pressures only and should be designed as such. I would view this similarly to that of a wall put in front of a rock face.

http://www.ce.utexas.edu/em2000/papers/AMSanad.pdf

http://www.omafra.gov.on.ca/english/engineer/facts/88-069.htm

http://www.jenike.com/pages/education/papers/silo-failures.pdf

"Earth Pressures on Unyielding Retaining Walls of Narrow Backfill Width", Canadian Geotechnical Journal, No. 6, Dec 2001.

 

[Geo69]

Thanks BigH.

Based on calculations, I've previously estimated wall pressures for this case by indicating that the soil pressure will increase down to a depth of two times the distance between the foundation wall and retaining wall. The pressure will stay constant below that depth. I've done some calculations using both active and at-rest pressures that indicate this. I've compared these calculations to the pressure based on a silo and they appear to have some agreement.

This is one of those things that remind me geotechnical engineering is an art as well as a science!

 

[BigH]

I also ran into the following thesis:

http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape17/PQDD_0010/MQ38414.pdf

 

[DRC1]

Many many years ago Civil Engineering magazine dicuseed reducing pressures due to inclined sheeting. The reduction of the lateral pressure was equal to the reduction in the area for the failure wedge for vertiacl sheeting to that of inclined sheeting, A similar anlogy might be made here.
Another alternative is to use a Culman's Diagram, with wedges stoping at the reaining wall.
My concern with the soil nail wall is if the nails will be bonded far enough outside the failure wedge if the retaining wall is in the failure wedge.

 

[PEinc]

DRC1,

That's the approach (area reduction) I usually look at first when I have a similar situation. Another example of that situation is where you are supporting dirt in front of a close building but are not going below the adjacent building.

 

[bharms1542]

Geo69

Have you ever checked out Redi-Rock. Its massive size and weight allow you to build closer to the property line and go higher without using geogrid in alot of cases. They also have a natural rock look which is aesthetically pleasing. Blocks range from 500-2000 lbs. each.